The construction and manufacture of solid electrolytic capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide and intrinsically conductive polymers such as polyaniline, polypyrrole, polythiophene etc. The solid cathode electrolyte is applied so that it covers all dielectric surfaces. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. In addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically consists of several layers which are external to the body of the porous or etched anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such as solder or a silver adhesive; and a highly conductive metal lead frame. It is important that the solid electrolyte be of sufficient buildup and density to prevent the overlaying layers from penetrating the solid electrolyte and contacting the dielectric. The reason for this is that these outer layers do not exhibit the healing properties required for a material directly in contact with the dielectric. Thus the ability to control the buildup, morphology, uniformity, and density of the solid electrolyte is critical to manufacturing a reliable solid electrolytic capacitor. The various layers of the external cathode also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
In the case of conductive polymer cathodes the conductive polymer is typically applied by chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The carbon layer serves as a chemical buffer between the solid electrolyte and the silver layer. Critical properties of the carbon layer include adhesion to the underlying layer, wetting of the underlying layer, penetration of the underlying layer, bulk conductivity, interfacial resistance, compatibility with the silver layer, buildup, and mechanical properties.
The silver layer serves to conduct current from the lead frame to the anode and around the anode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and mechanical properties. Compatibility with the subsequent layers employed in the assembly and encapsulation of the capacitor are also critical.
An electrically conductive adhesive is used to attach the silver layer to a lead frame. The electrical properties of the capacitor can be affected if the mechanical integrity of the adhesive/lead frame connection is degraded during assembly and post assembly processing. The adhesive properties of the conductive adhesive, the solder coating on the lead frame, the surface characteristics of lead frame, the coefficient of thermal expansion of the lead frame etc, are need to be carefully controlled in order to obtain durable negative connection integrity. The adhesive/lead frame interface is subjected to various thermomechanical stresses during molding, curing, aging, surface mount testing, solder reflow etc. These thermomechanical stresses and the low adhesive strength of the conductive adhesive cause stresses and lead to negative lead breaks. Adhesives with higher adhesive strengths and lower concentration of conductive particles are able to withstand the stress and maintain mechanical integrity. However, there is trade-off between increasing adhesion and increasing electrical conductivity.
Conductive adhesives are heavily filled with silver particles to get maximum conductivity. Increasing the silver loading decreases binder/resin concentration in the adhesive. The adhesion is contributed by the binder/resin portion of the adhesive. Increasing the resin portion will increase adhesion but decrease electrical properties. Improving electrical properties by increasing silver content will significantly decrease adhesive property of the conductive adhesive.
U.S. Pat. No. 6,972,943 attempts to circumvent the conflict between adhesion and conductivity of the adhesive by modifying the lead frame surface. The method of the invention in the patent provides grooves and holes in the lead frame so as to have good mechanical integrity between the two surfaces.
U.S. Pat. No. 6,916,433 attempts to improve performance by using conductive fillers with dendrites or protrusions to enhance contact with electrodes and an elastic adhesive resin for enhanced flexibility. The preferred elastic adhesive is a thermosetting resin comprising denatured silicon resin with a dispersed epoxy resin, available from Cemedyne Co. Ltd.
Through diligent research the present inventors have developed method of improving adhesive strength between the lead frame and the cathode layer which circumvents the problems encountered using the prior art adhesives and fillers.